1. Field of the Invention
The present invention relates to a process for the production of a grain oriented electrical steel sheet used as an iron core of an electric device. More particularly, the present invention relates to a process in which the slab-heating temperature is lower than 1200.degree. C., i.e., a production process in which an inhibitor is formed after the completion of cold rolling, where a product having a high flux density can be prepared even from a material having a high Si content.
2. Description of the Prior Art
A grain oriented electrical steel sheet is composed of crystal grains having a Goss orientation having a &lt;001&gt; axis in the rolling direction on the {110} plane [expressed as orientation {110}&lt;001&gt; by Miller indices], and is used as a soft magnetic material for an iron core of a transformer or electric appliance.
This steel sheet should have excellent magnetic characteristics, such as magnetization and iron loss characteristics, but whether or not the magnetization characteristics are good depends on the density of the magnetic flux induced in an iron core under the magnetic field applied, and if a product having a high flux density (grain oriented electrical steel sheet) is used, the size of the iron core can be diminished.
A steel sheet having a high flux density can be obtained by an optimum arrangement of the orientation of crystal grains in {110}&lt;001&gt;.
The term, iron loss, refers to the loss of power consumed as heat energy when an alternating magnetic field is applied to the iron core, and whether or not the iron loss characteristic is good depends on the flux density, the sheet thickness, the impurity content in the steel, the resistivity, the crystal grain size, and the like.
A steel sheet having a high flux density is preferred because the size of the iron core of an electric appliance can be diminished and the iron loss can be reduced, and therefore, development of a process for preparing a product having as high a flux density as possible, at a low cost, is urgently required in the art.
A grain oriented electrical steel sheet is prepared by the secondary recrystallization process, in which a hot-rolled sheet obtained by hot-rolling a slab is subjected to an appropriate combination of cold rolling and annealing to form a steel sheet having a final thickness, and the steel sheet subjected to finish annealing to selectively grow primary recrystallized grains having an orientation {110}&lt;001&gt;, i.e., secondary recrystallization.
The presence of fine precipitates, for example, MnS, AlN, MnSe, (Al, Si)N, and Cu.sub.2 S, and intergranular elements such as Sn and Sb in the steel sheet before secondary recrystallization is indispensable for the attainment of a secondary recrystallization. As explained by J. E. May and D. Turnbull [Trans. Met. Soc. AIME 212 (1958), pages 769-781], these precipitates and intergranular elements exert a function of selectively growing grains having an orientation {110}&lt;001&gt; while controlling the growth of primary recrystallized grains in an azimuth other than the orientation {110}&lt;001&gt; at the finish annealing step.
This effect of controlling the growth of grains is generally called the inhibitor effect.
Accordingly, a serious problem in the research in the art is how to clarify what precipitate or intergranular element should be used for stabilizing a secondary recrystallization, or how an appropriate presence state of the precipitate or intergranular element should be attained for increasing the presence ratio of grains having a precise orientation {110}&lt;001&gt;.
Since a high degree of control of the orientation {110}&lt;001&gt; is limited by the use of one kind of precipitate, development of a technique for preparing a product having a high flux density, stably and at a low cost, is now under serious study, and the merits and demerits of various precipitates and an organical combination of several precipitates are being examined.
Regarding the kind of precipitates, MnS is reported by N. F. Littmann in Japanese Examined Patent Publication No. 30-3651 and J. E. May and D. Turnbull in Trans Met Soc. AIME 221 (1958), pages 769-781, AlN and MnS are reported by Taguchi and Sakakura in Japanese Examined Patent Publication No. 33-4710, VN is reported by Fiedler in Trans. Met. Soc. AIME 212 (1961), pages 1201-1205, MnSe and Sb are reported by Imanaka et al in Japanese Examined Patent Publication No. 51-13469, AlN and copper sulfide are reported by J. A. Salsgiver et al in Japanese Examined Patent Publication No. 57-45818, and (Al, Si)N is reported by Komatsu et al in Japanese Examined Patent Publication No. 62-45285. Furthermore, TiS, CrS, CrC, NbC and SiO.sub.2 are known.
As the intergranular element, As, Sn and Sb are reported by Tatsuo Saito in Journal of the Japan Institute of Metals, 27 (1963), page 186, but these elements are not used alone in the industrial production and are used in combination with precipitates, with a view to attaining an auxiliary effect.
Characteristic inhibitors are disclosed by H. Grenoble in U.S. Pat. No. 3,905,842 (1975) and by H. Fiedler in U.S. Pat. No. 3,905,843 (1975). Namely, the production of a grain oriented electrical steel sheet having a high flux density is made possible by the presence of an appropriate amount of solid-dissolved S, B and N.
The standard for selection of a precipitate effective for the secondary recrystallization has not been completely clarified, but a typical opinion is stated by Matsuoka in, Iron and Steel, 53 (1967), pages 1007-1023. This opinion is summarized below.
(1) The size should be about 0.1 .mu.m. PA1 (2) The necessary volume is at least 0.1% by volume. PA1 (3) The precipitate should not be completely dissolved or should not be completely insoluble in the secondary recrystallization temperature but should be solid-soluble to an appropriate extent.
The above-mentioned various precipitates satisfy some but not all of these requirements. In the process of the present invention, where the steel plate is nitrided after the cold-rolling step, the requirement (1) is of no significance.
As pointed out hereinbefore, a guidance principle for selection of a precipitate has not been established, and a search for a new technique for controlling an inhibitor has been made by trial and error.
To obtain a high flux density [high integration degree of orientation {110}&lt;001&gt;], a large quantity of a fine and uniform precipitate must be present in a steel plate before finish annealing, and the properties before the secondary recrystallization must be adjusted by not only control of the precipitate but also an appropriate combination of the rolling and heat treatment in compliance with the characteristics of the precipitate.
Three typical processes are now adopted for the industrial production of unidirectional electromagnetic steels, and each has merits and demerits.
The first process is a two stage cold rolling process using MnS as the inhibitor, which is proposed by M. F. Littmann in Japanese Examined Patent Publication No. 30-3651. According to this process, secondary recrystallized grains are stably grown, but a product having a high flux density cannot be obtained.
The second process is a one stage cold rolling process in which (AlN+MnS) is used as the inhibitor and final cold rolling is carried out under a high reduction ratio exceeding 80%, as proposed by Taguchi and Sakakura in Japanese Examined Patent Publication No. 40-15644. According to this process, a product having a very high flux density can be obtained, but in industrial production, the preparation conditions must be strictly controlled.
The third process is a two stage cold rolling process in which [MnS (and/or MnSe)+Sb] is used as the inhibitor, as proposed by Imanaka et al in Japanese Examined Patent Publication No. 51-13461. According to this process, a relatively high flux density can be obtained, but since poisonous and expensive elements such as Sb and Se are used, and cold rolling is conducted twice, the manufacturing cost is high.
These three processes have the following problem in common. Namely, in each of these processes, to form a fine and uniform precipitate, the precipitate must be once solid-dissolved, and therefore, the slab-heating temperature must be high.
Note, in the first process the slab-heating temperature is higher than 1260.degree. C., and in the second process, as disclosed in Japanese Unexamined Patent Publication No. 48-51852, the slab-heating temperature differs according to the Si content in the material: where the Si content is 3%, the slab-heating temperature is 1350.degree. C. In the third process, as taught Unexamined Patent Publication No. 51-20716, the slab-heating temperature is higher than 1230.degree. C., and in the example where a high flux density is obtained, the slab-heating temperature is as high as 1320.degree. C.
Namely, a slab is heated at a high temperature to solid-dissolve the precipitate and is precipitated again during the subsequent hot-rolling or heat-treating step.
Since the slab-heating temperature is high, the consumption of energy for heating is increased and the yield is reduced by slag formation. Moreover, problems arise such as an increase of the cost of repairing a heating furnace and reduction of the operation rate of the equipment. Furthermore, as taught in Japanese Examined Patent Publication No. 57-41526, a linear secondary recrystallization-insufficient portion is formed if the slab-heating temperature is high, and therefore, a continuously cast slab cannot be used.
In addition to the above-mentioned cost problem, there is another serious problem. Namely, if an iron loss-reducing means such as an increase of the Si content or reduction of the thickness of the product is adopted, the above-mentioned linear secondary recrystallization-insufficient portion is conspicuously formed and future improvement of the iron loss characteristics cannot be gained in the process in which a slab must be heated at a high temperature.
As a means for solving such problems, Japanese Examined Patent Publication No. 61-60896 proposes a process in which the secondary recrystallization is greatly stabilized by reducing the S content in steel, and an increase of the Si content and a reduction of the thickness become possible.
Furthermore, there can be mentioned a process proposed by H. Grenoble in U.S. Pat. No. 3,905,842 and a process proposed by H. Fiedler in U.S. Pat. No. 3,905,843. These processes, however, include substantial contradictions and are not industrially worked. Namely, according to this technique, since the inhibitor is composed mainly of solid-dissolved S, to maintain solid-dissolved S, the Mn content must be reduced so as not to form MnS. More specifically, a requirement of Mn/S.ltoreq.2.1 must be satisfied. But, as is well-known, solid-dissolved S has a bad influence on the toughness of the material, and accordingly, in the unidirectional electromagnetic steel plate which has a high Si content and is easily cracked, it is very difficult in industrial production to cold-roll a material containing such solid-dissolved S.
As pointed out hereinbefore, to make it possible to produce a thin product having a high flux density and a high Si content, in which a reduction of the iron loss will be possible in the future, a reconstruction of the inhibitor design is necessary.